Since our $15 "play money" ran out, I disabled using SimpleCDN for now. I have been monitoring our server CPU loads using Cacti, and there has been NO change in loads since we started serving images from our own server again. BUT, I am positive we would still see a result due to decreased disk I/O and a few CPU cycles, so we may revisit this in the near future when we raise the funds to pay into it. Our $15 lasts at least six weeks, maybe more.
All the while, though, I must say that SimpleCDN worked flawlessly. And my replacement variables are easy to reinstall, so I can be serving images from them again with a minimum of work.
I just hope vB4 lets us change things this easily.